This article was originally published in the November/December 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.

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Home Energy Magazine Online November/December 1996

Heat Pumps and Manufactured Homes:
Making the Marriage Work

by Francis Conlin with C. Leon Neal

Francis Conlin and C. Leon Neal are engineers at the North Carolina Alternative Energy Corporation in Raleigh, North Carolina.

Manufactured homes make up over 7% of the U.S. housing stock, including over 15% of the homes in North Carolina. As more of these homes are being equipped with heat pumps, it becomes important to figure out how to make these systems efficient. A 1995 study in North Carolina found a variety of ways to do just that.

The author demonstrates that air leaks and other energy-related problems aren't the only things that contractors encounter under manufactured homes!

In my job as the factory-built home specialist for North Carolina's Alternative Energy Corporation (AEC), I have been crawling under, in, and around manufactured homes for the last dozen years, looking for causes of, and solutions to, building energy problems. I have seen lots of things. I even have a series of wildlife photographs taken from the special environment found only underneath these homes. Perhaps, I have speculated, it's this wildlife that makes it so hard for contractors to deliver quality work on manufactured homes. There is, indeed, nothing quite like lying on your back amidst rural North Carolina's abundant flora and fauna, with 10 tons of home jacked up above you. Contractors watching for snakes and other common crawly creatures may be tempted to cut corners. Recently, however, one animal that I had seen only rarely before around manufactured homes has begun to appear in abundance-the Heat Pump.

Heat pumps are ideal for manufactured homes in the Southeast. Since little natural gas is available in rural areas, almost all new manufactured homes leave the factory with an electric furnace. When the home is purchased, a central air conditioner is often included as part of the package. The air conditioner is actually installed after the home is set up on site. Because of the Southeast's low heating loads, it often makes sense for a home buyer to get a heat pump, rather than central air conditioning, when buying a manufactured home. When a heat pump is installed, the existing furnace is left in place. Its air handler is used for the heat pump and its coils are used for backup heat. The higher cost of the heat pump is easily recovered in the first year and a half by savings from reduced electric resistance heat. According to North Carolina electric utilities, between 1994 and 1995, sales of manufactured homes with heat pump package deals went up by 250%.

Multi-section homes have a marriage wall between the sections. This wall is often poorly sealed, so it can fill with outside air, making it respond to outdoor temperature even more than insulated exterior walls. A thermostat on a leaky marriage wall won't respond to the inside temperature, and will likely be operated as an on/off switch. This is an annoyance to the homeowner, and is bad for the heat pump-a heat pump thermostat that senses too much heating load will turn on wasteful backup resistance heating elements.

The Manufactured Homes Study
In 1995, as part of a four-state study on manufactured homes, I studied seven new multisection manufactured homes that had been equipped with heat pumps. Carolina Power and Light Company conducted an instantaneous-performance test on these heat pumps. The AEC had previously done a similar study for site-built homes (see Air Conditioner Efficiency in the Real World, HE May/June '92, p. 32), which demonstrated that the actual performance of air conditioners and heat pumps was as low as 26% of the performance rated by the Air Conditioning and Refrigeration Institute (ARI). The inefficiencies were primarily due to installation and service problems. We found similar problems in manufactured homes.
Actual and Rated Capacity
Heat pump capacity is rated by ARI at standard outdoor temperatures of 47oF for heating and 95oF for cooling. In the field, these temperatures are not often conveniently available. However, performance data under local conditions can be adjusted by formulas to compare to ARI ratings, with a margin of error of 10%. Of the equipment that could be field-tested for performance, only one unit out of five was satisfactory. The others had significant performance degradation, due to either poor air flow across the coil, improper charge, or some combination of both. Adjusting for the conditions at testing, the average measured heating or cooling capacity of installed units was only 78% of their rated capacities (see Table 1).

Heat Pump Sizing
Improperly sized air conditioning equipment causes many problems (see Sizing Air Conditioners: If Bigger Is Not Better, What Is? HE Sept/Oct '96, p. 13). The most notable of these is that oversized units cause shorter run times, which result in poor efficiency, and, in humid climates, poor dehumidification. Individual heat loss and gain calculations were not carried out for each house in this sample. However, all the homes were similarly insulated as required by the HUD Code (see Manufactured Housing-an Evolutionary Home), were located within 60 miles of Raleigh, North Carolina, and had window areas equal to 8%-11% of floor areas. The Air Conditioning Contractors' Association Manual J calculation for similar homes with various orientations and worst-case window orientations specifies 1 ton of cooling for every 600 to 800 ft2 of floor area. Only one of the field study heat pumps fell within this size range; the rest were oversized by 18%-60%.

Table 1. Field Performance of Five Heat Pumps in North Carolina Manufactured Homes.

Test Site

Home A

Home B

Home C

Home D

Home E

Target

Floor area (ft2)

1,266

956

1,677

1,460

2,083

Outdoor unit rated capacity

3 tons

3 tons

3.5 tons

3 tons

3.5 tons

Ratio of floor area to equipment size (ft2/ton)

420

320

480

490

600

600-800

Air flow to the coil (CFM/ton)

380

290

400

230

200

400

Duct leakage to exterior (CFM at 25 Pa)

270

150

140

130

320

5% of floor area

(target)

(63)

(48)

(84)

(73)

(104)

Test mode

Heating

Cooling

Cooling

Heating

Cooling

Capacity in field as % of rated capacity

103%

78%

60%

69%

78%

100%

Note: All homes were less than one year old, and were built to the October 1994 HUD Code.

Manufactured Housing-an Evolutionary Home

The ancestor of today's manufactured home is the house trailer. It was developed after the Second World War, was designed to be towed from place to place behind a car, and was a boon to a shelter-hungry country. It proved to be a surprisingly durable home, and it is not uncommon to find no-longer-roadworthy trailers still functioning far beyond their design life.

In the 1950s, house trailers became 10 ft wide and it took special trucks to move them from place to place. They came to be called mobile homes. Many manufacturers were competing in this industry, and quality was sometimes compromised. Some states felt that if mobile homes were going to be used as permanent housing, they should be regulated with fire and safety standards. Several states began specifying their own standards for mobile homes.

This presented a particular problem for the mobile home industry: homes built to the standards of one state would often not meet the codes in another state. What is the point of being a mobile home if you can't go anywhere? Also, one of the benefits of mass production would be lost if the factory had to produce each home to satisfy a different local code.

In 1976, the U.S. Department of Housing and Urban Development (HUD) began administering standards for manufactured homes. The Manufactured Housing Construction and Safety Standards Act (often referred to as the HUD Code) set what seemed, for the time, reasonable but aggressive standards for these homes. In turn, the mobile home industry got an exemption from state code jurisdiction. These standards changed the industry. In fact, they were so challenging that the number of mobile homes sold dropped over 40% between 1974 and 1976.

The industry has since tried to differentiate homes built to the HUD Code from the older pre-HUD Code homes by officially calling new units manufactured homes. This change was also intended to move away from the implications of transiency inherent in the term mobile homes. And, in fact, more of the homes were being set up on permanent foundations and were no longer really mobile. The term mobile home, however, has not obliged by going away. It is still commonly used both within and outside of the industry. Since the 1970s, the trend in the manufactured housing industry is towards multisectional, custom-ordered homes with removable axles. However, there are still many single-section homes being built, and some of these are lower end homes that remind us of the links with trailers and mobile homes of the past.

The HUD Code was updated in October 1994. This long-overdue change significantly increased, among other things, the thermal insulation and wind resistance requirements. This time, the accompanying increase in costs did not scare away buyers. In 1995, 340,000 HUD Code homes were sold, double the 1991 sales, and the most sales since the original HUD Code was enacted.

This picture was taken in a two-month-old manufactured home, looking through the crossover boot towards the furnace. Although the home is new, there is already a major failure in the duct where the finger joints have separated from the main supply plenum. Because this is the point of highest pressure in the system, an air leak here is quite severe.

Air Flow to the Coil
Proper air flow across the indoor coil is critical for good heat pump performance. The standard air-flow requirement for the indoor coil is 400 cubic feet per minute (CFM) per ton of cooling. In the study houses, air flow was calculated using the temperature difference measured across the coil and the amount of heat added by the strip heaters. Only three of the five systems were operating within the acceptable 350-450 CFM/ton range. The one system with properly sized equipment had air flow that was 43% too low.

Like most manufactured homes that are being equipped with heat pumps, the units had factory-equipped electric furnaces with four-speed blowers. The typical four-speed blower provides 910, 1,130, 1,260, and 1,400 CFM standard air flow at 0.2 inches of water column static pressure. Measured static pressure was generally close to 0.2 inches, and never exceeded 0.26. Thus, proper air-flow capacity for these 3- to 3.5-ton heat pumps was available. However, in most cases, the blower speed was not changed when the systems were converted from electric furnaces to heat pumps.

Indoor Thermostat Placement
To save money at the factory, a thermostat control cable with only two wires is installed with the electric furnace, but a heat pump thermostat often requires an eight-wire control cable. If a heat pump installer is required to run new thermostat wiring, to avoid crawling under the house (remember the creepy crawlers) she or he will frequently put the thermostat in the most convenient spot to wire, not in a logical spot for heating and cooling purposes.

One test home's new heat pump thermostat was installed in the kitchen adjacent to the air handler closet, directly over a supply grille. Of course, it was easily installed from inside the house; but every time the equipment operated, the supply air blew directly on the thermostat, quickly shutting the system off. This system was destined to an entire lifetime (albeit a short one) of start-and-stop operation.

The polyethylene air barrier under this mobile home had large holes torn in it during a telephone wire installation. Running the wires required a space of about 14 inch, but messy work resulted in 18-inch-square holes, a serious breach of the air barrier. This wire runs between two sections of a multi-unit home, disrupting air sealing in both parts of the house.

Outdoor Thermostat
A desirable control for heat pumps is an outdoor thermostat that prevents the operation of the resistance heat when the heat pump can handle the load. These lockout outdoor thermostats need to be set at an appropriate temperature, above which the resistance heat won't be used. This study found one lockout set at 55oF, when it should have been set closer to the balance point of 35oF-40oF.

Condensate
All of the heat pumps examined had prefabricated condensate line traps. However, in order to route the condensate from the trap to the outside, the pipe connected to the trap was sometimes angled downward. Traps that are not level will leak outside air into the equipment, and will often cause water in the drip pan to overflow. Some new equipment has an internal condensate trap that should prevent shoddy installations in the field.

Leaky Ducts
Several Home Energy articles have challenged the practice of installing high-efficiency air conditioning equipment into site-built homes when the duct system performs so poorly that the effective efficiency of the equipment is seriously reduced. No one has checked whether this principle holds true in the new manufactured home designs. However, another recent study of manufactured homes showed air distribution systems were responsible for losing 40% of HVAC systems' heating efficiency and 18% of their cooling efficiency.

A supply leak fraction of 14% was found in the North Carolina homes. This means that 14% of the air coming out of the air handler was delivered somewhere other than to the supply registers. Because of the floor design, some of this conditioned air entered the living space through holes in the floor, and additional energy was recovered by conditioning the floor.

Besides ubiquitous random holes in the ducts throughout the belly of the home, duct leaks also result from poor crossover connections (see Duct Improvement in the Northwest, HE Jan/Feb '96, p. 27). The catastrophic failure where the crossover duct has fallen away from the in-floor plenum is usually discovered by the observant homeowner, who notices the lack of climate control on one side of the home or receives a shocking utility bill. Less obvious leaks, where only a portion of the crossover air is directed into the crawlspace, may go unnoticed for years, until someone crawling under the home sees spiderwebs waving in the leaking air, or until the ductwork is tested.

A manufactured home undergoes duct tightness testing while under construction. Manufacturers consistently do a better job of quality control than retailers and installers, so a manufactured home benefits greatly when most work is done at the factory. To fully benefit from this early testing, it is equally important to check the home at the building site for problems resulting from either transit or on-site assembly.

Pressure Boundary Breaches
Southern manufacturers assume that these homes have a warm floor design, meaning that the thermal boundary is on the outermost layer of the floor construction. The bottom side of the floor is wrapped in a blanket of insulation and protected from the outside with a continuous woven polyethylene road barrier. This barrier is also intended to serve as the pressure boundary. Leaky ducts have not been considered very seriously, because theoretically they are within the home's thermal shell, in the area known as the belly.

In practice, however, many breaches in the air barrier are common, and these defeat the design scheme. The setup crew can meticulously repair the inevitable holes torn in transit and setup. Still, other contractors-including HVAC installers-anxious to get out from beneath the home have been known to cut large holes in the air barrier to install a single wire. In another AEC study, we found that the bellies of manufactured homes had an average 29 Pa pressurization with respect to the inside during 50 Pa depressurization tests. Patching holes can help the air barrier work.

However, patching is not all that's needed. In one home where our crews repaired every visible hole in the belly (including a 2 ft x 4 ft hole made by the cable TV installer), the floor cavity was still communicating with the outside. A pressure probe placed in the floor cavity during a 50 Pa depressurization test showed an average belly pressure of 14 Pa with respect to the inside. No one has conducted a complete test to determine how air gets around the air barrier, but one explanation is that it leaks through gaps between the staples that hold the perimeter of the air barrier to the home. In fact, the whole design of this external air barrier is so prone to problems that in some areas, such as the Pacific Northwest, weatherizers and manufacturers alike have accepted that this sheet of fabric is a road barrier, not an air barrier.

Heat Pumps-What Should You Look for?

Energy professionals working on manufactured homes need to check out the installation, technical support, and service of the heat pumps. Here are some of the key items to check.

Be sure the equipment is properly sized. Retailers often specify oversized units based upon previous experience with callbacks.

Check the blower speed-usually a wire will be connected to the low, med, med-high, or high connector on the air handler motor. Find the installation instructions for the air handler and see how much air flow this setting is supposed to provide. If it is not between 350 and 450 CFM per ton, check the static pressure in the duct and set the blower speed to within this range.

Assess the thermostat location. It should be in the return airstream, and it should not be influenced by nearby supply grilles or heat sources such as lights, windows, or other appliances. It should be on an interior wall, but if possible, not on the marriage wall.

Check the outdoor thermostat. It should be installed, properly connected, and set to 35oF-40oF.

Check the condensate line to see that it is properly situated. When the equipment is cooling, the condensate line should be dripping outside the crawlspace. If it is not visibly dripping, it is not working properly.

Make sure there are no holes in the air barrier. I seal smaller holes with expanding foam. For larger holes, I put a patch inside the belly, stitch it with a stapler, and seal it with mastic. (For really big holes, you have to get more creative.)

Make sure the ducts are sealed, checking for duct leaks with a blower door or Duct Blaster. Ducts sealed with aluminum tape will eventually fail, due to the film of oil that is usually left on the raw duct material. If you see aluminum tape, in the riser boots, for example, you can bet that the furnace and crossover ducts are also sealed with tape. Leaks at these sites can be large, and they can often only be seen and repaired from underneath the home.

A quality service technician is key to a properly operating heat pump. The manufactured home retailer may not be able to offer the quality that residents need, since retailers often employ technicians who have been trained only in installation.

Pressure Problems
Duct leakage always causes unwanted induced pressure differentials when the air handler is operating. Manufactured homes typically have no return ducts, so leaks usually induce negative pressures, which drag in outside air and increase heating and cooling loads. Problems in homes with leaky ducts can be masked by oversizing the cooling equipment. Recall the problems with oversized equipment. The prevalent duct problems have convinced some retailers for life that their rule of thumb for equipment sizing is better than using any load calculation procedure.

But They Can Work Fine!
Heat pump systems that work properly can provide affordable space conditioning to manufactured homes. In a demonstration project conducted by AEC several years ago, five occupied manufactured homes conditioned with heat pumps were monitored for energy use. In these homes, many potential problems, such as duct leakage, proper equipment sizing, and holes in the air barrier, were repaired. The homes had remarkably little energy use for climate control. The annual consumption by the heat pumps in these new homes was less than 5,000 kWh. At 8¢ per kWh, this would cost the homeowner $33 per month.

As manufactured housing competes for a bigger chunk of the new housing market, the general quality of the product seems to be on the rise. More manufacturers are paying attention to understanding and reducing performance problems such as duct leakage. Several states are also increasing the role that their local code inspectors play in manufactured home setups. The incidence of inferior home installations should decrease as code officials become trained to enforce minimum home setup standards.

Manufacturers typically do a better job of quality control than retailers and installers. The more work that is done at the factory, the more reliable the quality tends to be. Some manufacturers offer a heat pump ready option which ensures that extras like the eight-wire thermostat control cable and a multispeed air handler motor are part of the home specifications. Other manufacturers have begun to offer a heat pump that can be installed in the factory. As more heat pumps are installed in manufactured homes from the start, many installation problems should become a thing of the past.

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